(1) Field of the Invention
The present invention relates to self-oscillating pulse width modulators, using a hysteretic comparator to change the output duty cycle according to the input signal.
(2) Description of Prior Art
A pulse width modulator (PMW) creates an output square wave where the duty cycle depends on its Input signal. PWM's are widely used within e.g. Class-D amplifiers or switching voltage regulators, where the output load is switched either to the negative or the positive supply, by simple MOSFET switches. Using a self oscillating (or hysteretic) modulator type results in very low noise and distortion values. However, compared to the conventional PWM type with fixed (external) clock, the switching frequency is not constant, but varies significantly with input signal amplitude, output power and with supply voltage.
FIG. 1 shows one conceptual circuit diagram of a self-oscillating pulse width modulator and FIG. 2 demonstrates as one example the variation of frequency with large amplitude of a sinusoidal signal input (Vin). The moving frequency (Fp) can create several problems: with large input signal amplitude, i.e. per high modulation depths Fp becomes very low and may interfere with the (audio) signal. Further the switching noise created on the supply line disturbs other circuits and can't be filtered effectively, due to the wide frequency range. Such standard hysteretic PWM's of prior art are described in all patent references mentioned below.
U.S. Pat. No. 5,160,896 (to David McCorkle) describes a circuit to limit or regulate the switching frequency, e.g. by modifying the comparator hysteretis dependent on the Input voltage, using an analog multiplier to provide a varying hysteretic voltage.
U.S. Pat. No. 6,107,875 (to Stuart Pullen, et al.) describes a variable Class-D modulator that uses gain compression to ensure that switching frequency never falls below a minimum target.
U.S. Pat. No. 6,297,693 (to Stuart Pullen) discloses a circuit using an amplifier that synchronizes an external clock input the summing node of the integrator, an amplifier that gates an external clock to the modulator and an amplifier that adjusts its own hysteretic.